Galileo Galilei (Pisa, 1564 – Arcetri, 1642), a mathematician at the University of Padua, points a telescope at the sky in January 1610 and discovers Jupiter's four main satellites — today called the Galilean moons —, mountains on the Moon, the stellar nature of the Milky Way, and the phases of Venus. He publishes the results in Sidereus Nuncius (March 1610), the first astronomical work based on telescopic observation. The phases of Venus are direct proof that Venus orbits the Sun, incompatible with Ptolemy's geocentric model. In 1616 the Inquisition formally declares heliocentrism heretical; in 1633 Galileo is tried and sentenced to recite the penitential psalms weekly. He remains under house arrest until his death. Galileo's institutional condemnation is the most cited historical case of suppression of scientific knowledge by religious authority. Newton explicitly cites his work on kinematics in the Principia.